The light that has been incident on a lens will have a refractive index that varies according to its wavelength while passing through the material of that lens. That is why if multiple light beams with mutually different wavelengths have been incident on an optical system for an image capture device, a longitudinal chromatic aberration will be produced. As a result, the sharpness of the resultant image will vary from one color to another. And if there is any color with a low degree of sharpness in the image, then that color becomes a factor in debased image quality.
Meanwhile, if the subject is located within the range of the depth of field, an image capture device such a camera can capture a sharp image by focusing right on that subject. And to capture a subject that can be located at any of various positions, the image capture device needs focusing state detecting means and focus adjusting means.
In order to overcome these problems, someone proposed a technique for extending the depth of field and correcting the longitudinal chromatic aberration at the same time by using the longitudinal chromatic aberration of an optical system so that the sharpness of a first color component is reflected on a second color component, which is different from the first color component (see Patent Document No. 1). According to the method disclosed in Patent Document No. 1, the sharpness of the first color component is reflected on the second color component, thereby increasing the sharpness of the second color component. As a result, the depth of field can be extended, and a subject that may be located at any of various distances can be captured relatively sharply even without making a focus adjustment.